Typically, device structures are formed on one face of a substrate such as a semiconductor wafer. The substrate may typically be a flat tabular body such as a flat disc with opposed faces bounded by an edge or edges.
In order to observe the device structures on such a substrate, it is possible to cleave or cut through the sample to provide a cross-sectional facet, typically at right angles to the opposed faces of the sample. Mechanical cutting and polishing leaves marks and striations which are on too large a scale for techniques such as scanning probe microscopy to be effective. Cleavage may be successful for mono-crystalline structures, but may result in ladder faulting for hetero-structures, again leading to problems when carrying out scanning probe microscopy.
Ion beam polishers are known in the prior art. U.S. Pat. No. 5,907,157 and US2005/0081997A1 disclose methods and apparatus for preparing samples for electron microscopy where an edge of a sample is milled by ion beam milling to give a smooth cross-sectional profile. A mask is used to define a boundary between irradiated and un-irradiated regions and the mask is positioned over a face of the sample so that the polished edge formed by the ion beam removing a portion of the sample is at right angles to the face upon which the mask rests.
Such an arrangement is shown in FIG. 1 of the present specification and will be described in more detail hereinafter.
One problem with such a prior art arrangement is that the ion beam inevitably damages the leading part of the polished facet resulting from ion beam milling (i.e. the part of the facet upon which the ion beam first impinges). This arises from scattering of ions around the mask edge and possibly their partial implantation into the sample. Moreover, it has now been found that the ion beam may create a bevel or round-nosed profile at the leading part of the ion-beam milled edge. Such a bevel may typically extend for several micrometers. This bevel precludes high resolution investigation of surface layers of interest that are in the region adjacent to the surface. In particular, when surface probe microscopy is subsequently carried out, severe scanning problems may result.
Although this problem may be overcome in part by turning the sample over, and using the back of the substrate as the leading part for ion beam milling, it will be necessary to remove considerably more sample in order to polish the device layers on the trailing part of the edge. This may require a considerable length of time and may result in the need to frequently replace sample holders and masks. Furthermore, the area of interest of sub-micrometer dimensions will also be close to the 90 degrees edge. This may lead to the creation of unbalanced forces on the scanning microscope and is known to deteriorate images, especially for images generated in the ambient environment where a water meniscus between the probe and the sample might be present.
Hence there is a need for apparatus and methods which address some or all of the problems in the prior art as set out above.